1. Field of the Invention.
The present invention relates to switches in general and particularly to manual on-off switches for controlling hazardous machinery.
2. Background Art.
Rules and regulations governing design and operation of machinery often specify safety features including special electrical control systems designed to protect workers and prevent accidents. Such safely features are required in many categories of woodworking machines, e.g. circular saws, routers and planers, and metalworking machines including drill presses, punch presses and shears as well as many other types. The following are examples of safety features often incorporated in machinery and sometimes demanded by law.
On-off switches, easily accessible from the operator's position.
Guards protecting the operator from hazards created by ingoing nip points, rotating parts, and various kinds of cutters and blades. Guards are often provided with limit switches which render the machine inoperative if they are opened or removed. Examples are blade guards on circular saws, guards protecting the daylight area on presses and covers guarding interchangeable gears on lathes.
Two hand tripping devices used to ensure that both hands are kept clear of dangerous areas during operation. Two-hand devices often consist of two palm switches which must be depressed simultaneously to start a work cycle.
No-voltage-trip systems. These prevent hazardous, uncontrolled restart of equipment when a remote, tripped circuit breaker is closed or when power is restored after a power failure.
Maximum torque circuit breakers. These usually sense the motor current and are designed to trip instantaneously if a motor is stalled or overloaded beyond a predetermined limit. This type of protection is sometimes used on rotating machinery such as drill presses or lathes which may snag onto an operator's hair or clothing during operation.
Braking systems serving to prevent hazardous coasting of a machine after the power is cut off. Electrical braking of AC motors is often accomplished by applying a brief pulse of DC to the field windings immediately as a machine is turned off.
Incorporating all or a majority of the above features in a machine usually requires several electrical control components such as on-off switches, a magnetic motor starter, a magnetic overload circuit breaker and a motor braking circuit etc.
Attempts have been made to combine some of the functions described above into a single assembly such as for example on-off switches combined with magnetic or thermal circuit breakers, or magnetically latching push-button switches. These types of switches have certain features in common with the switch according to the invention and are described in U.S. Pat. Nos. 1,839,629, 3,161,743 and 3,622,925. They were designed to operate loads such as e.g. clothes dryers and elevators and have push-buttons used to start a work cycle which is then shut off automatically by means of a thermostat, a timer or a limit switch.
A different category of hybrid safety devices, the self-protected motor starters, include a magnetic motor controller and an overload relay. These have separate magnetic circuits but usually just one set of contacts serving both the contactor and circuit-breaker functions. In the majority of cases operation is initiated by means of a push-button switch which serves to energize the solenoid in the motor controller, and a holding circuit serves to keep the starter closed after the push-button is released. The holding circuit incorporates one normally open contact and one or several normally closed contacts connected in series. One of these contacts may be a stop push-button while another may be operated by a timer, a counter or a limit switch in order to stop the process in case of malfunction or after a work cycle is completed.